The records of 113 patients dying at the Francis Delafield Hospital with documented reticulum cell sarcoma revealed six cases whose course terminated in a syndrome resembling acute leukemia. Their course was characterized by weakness, pallor, petechiae, hemorrhages and hepatosplenomegaly. The blood showed anemia, leukocytosis (white blood cell count 20,000 to 80,000/ cu.mm.) and thrombocytopenia (platelet count [unknown] 100,000/cu.mm.). Differential count in the blood and the bone marrow revealed a high percentage of immature cells (35 to 96 per cent). These were identified as reticulum cells in three patients, as myeloblasts in two and as monocytoid granulocytes in one. In all six patients, this explosive illness terminated in systemic infection or hemorrhage within two months. Therapy with 6-mercaptopurine, adrenal steroids, or both, gave no benefit.